1. Technical Field
The present invention relates to a line head controlling method for correcting an exposure spot shift to prevent degradation in image quality, and an image forming method.
2. Related Art
There is an LED-based line head as an exposure light source of an image forming apparatus. JP-A-5-261970 proposes an inventive circuit that corrects an exposure spot shift in the direction in which a photoconductor rotates (secondary scan direction), the exposure spot shift caused by an LED line head having light emitters disposed in a staggered arrangement. In this inventive circuit, odd-numbered data and even-numbered data are separated and written in odd-numbered and even-numbered frame memories, respectively. In this process, the even-numbered and odd-numbered data are stored at different write addresses, the difference corresponding to the shift in row between an odd-numbered light emitter row and an even-numbered light emitter row. The data are then successively read from the frame memories in synchronization with a single strobe signal (in synchronization with a line data cycle). In this way, an exposure spot shift between an odd-numbered dot and an even-numbered dot is corrected on a basis of an integral multiple of the exposure spot diameter (the diameter of a single dot).
In the example described in JP-A-5-261970, the exposure spot shift cannot be corrected in some cases, for example, in an electrophotographic printer using an intermediate transfer belt. Such a case will be described below with reference to FIGS. 14A and 14B, descriptive diagrams showing related art of the invention. In FIG. 14A, reference numeral 10 denotes a line head. Reference numeral 41 denotes a photoconductor. Reference numeral 50 denotes an intermediate transfer belt that runs between a drive roller 51 and a driven roller 52 (transfer roller) and rotates in the direction indicated by the arrow R. Reference character P denotes a recording sheet that is fed in the direction indicated by the arrow S and on which a toner image is transferred at the position of the transfer roller 52. In a typical electrophotographic printer using an intermediate transfer belt, the ratio of the speed at which the photoconductor 41 rotates to the speed at which the intermediate transfer belt 50 rotates, that is, the speed at which the drive roller 51 rotates, is changed to adjust the tension of the intermediate transfer belt so that there is no cyclic stripes (banding) when the toner image is transferred onto the recording sheet P.
In this process, the ratio of the speed at which the photoconductor 41 rotates to the speed at which the intermediate transfer belt 50 rotates, that is, the speed at which the drive roller 51 rotates, causes expansion or shrinkage of the image in the secondary scan direction (the direction in which the photoconductor rotates). In this case, the dot-to-dot pitch in the image in the secondary scan direction (exposure spot pitch) is not an integral multiple of the exposure spot diameter (the diameter of a single dot), that is, a non-integral multiple of the exposure spot diameter. FIG. 14A shows a case where the photoconductor 41 rotates slowly, whereas the drive roller 51 rotates fast. In this case, the intermediate transfer belt 50 is held under tension. FIG. 14B shows a case where the photoconductor 41 rotates fast, whereas the drive roller 51 rotates slowly. In this case, the intermediate transfer belt 50 has a slack Rx in tension.
In such a case, since the configuration described in JP-A-5-261970 only allows the exposure spot shift to be corrected on a basis of an integral multiple of the exposure spot diameter (the diameter of a single dot), the correction is imprecise when the exposure spot pitch in the secondary scan direction is a non-integral multiple of the exposure spot diameter. For example, when a single linear latent image is formed in the axial direction (primary scan direction) of the photoconductor, the fact that the decimal part of the non-integral multiple cannot be fully corrected causes minute steps in the direction in which the photoconductor rotates (secondary scan direction). The image quality is therefore disadvantageously degraded.
Further, depending on the precision at which the line head is mounted on an apparatus body, the exposure spot pitch becomes a non-integral multiple of the diameter of the exposure spot formed on an image carrier some cases, resulting in a positional shift of the exposure spot. Such a case will be described below with reference to FIGS. 11A and 11B, descriptive diagrams showing related art of the invention. FIG. 11A shows a case where the precision at which the line head 10 is mounted on the apparatus body is insufficient, whereas FIG. 11B shows a case where the precision at which the line head 10 is mounted on the apparatus body is sufficient.
In FIG. 11A, reference numeral 2 denotes a light emitter provided on a substrate. Reference numeral 3 denotes a light emitter row formed of a plurality of light emitters arranged in the axial direction of the photoconductor. In the example shown in FIG. 11A, three light emitter rows A to C, each of which forms a light emitter array, are formed in the direction in which the photoconductor rotates. Reference character Ta denotes an inter-light-emitter-row pitch between the light emitter rows A and B. Now, let L1 be the distance between the light emitter rows A and B, and L2 be the distance between the light emitter rows B and C. The following equation is satisfied: L2≠nL1 (n is an integer greater than one). That is, the inter-light-emitter-row pitch is not an integral multiple of the exposure spot diameter (the diameter of a single dot), but a non-integral multiple of the exposure spot diameter. FIG. 11B shows a case where L2 is equal to L1 so that the inter-light-emitter-row pitch between the light emitter rows A and B is equal to the inter-light-emitter-row pitch between the light emitter rows B and C.
As described above, when the inter-light-emitter-row pitch in the secondary scan direction of the photoconductor is not fixed, the pitch between exposure spots formed on the photoconductor is not an integral multiple of the exposure spot diameter. Such a case will be described below with reference to FIGS. 12A and 12B and FIGS. 13A and 13B, descriptive diagrams showing related art of the invention. FIG. 12A shows a case where the pitch between exposure spots 4 formed on the photoconductor is an integral multiple of the exposure spot diameter (W1=W2). Reference characters Aa, Ba, and Ca denote exposure spot rows. In this case, as shown in FIG. 12B, a linear latent image Ea is formed in the axial direction (direction X) of the photoconductor. The direction Y is the direction in which the photoconductor rotates.
FIG. 13A shows a case where the pitch between exposure spots 4 is a non-integral multiple of the exposure spot diameter (W2≠nW1, n is an integer greater than one). In this case, when a single linear latent image is formed in the axial direction (direction X) of the photoconductor, the decimal part of the non-integral multiple cannot be fully corrected. Therefore, as shown in FIG. 13B, a formed latent image Eb has minute steps in the secondary scan direction (direction Y). In this case, the image quality is disadvantageously degraded.